Wireless portable devices such as mobile phone and personal digital assistant have become popular due to the convenient properties thereof, and the thickness and size thereof have been remarkably reduced in these years. FIG. 21A and FIG. 21B are perspective view and side view, respectively, showing the basic structure of a typical, conventional wireless portable device, and FIG. 21C is an exploded perspective view showing each layer structure of a printed circuit board therein. In these figures, a four-layer circuit board is exemplified, wherein a package casing for receiving therein the printed circuit board is not depicted for simplification purpose.
The wireless portable device includes an antenna block 20, a feeding block 23, a radio circuit block 21, and a digital circuit block 23, which are mounted on the printed circuit board. The antenna bock 20 transmits and receives radio waves that are used as signals upon communication with a base station etc. The feeding block 23 delivers signals to the antenna bock 20, and transmits signals supplied through the antenna block 20 to the circuit blocks. The radio circuit block 21 processes signals that are transmitted or received through the antenna block 20. The digital circuit block 22 processes digital signals that are used for data processing. Generally, a multi-layered circuit board having multiple layers is used as the printed circuit board 10. The ground layer formed as an internal layer of the printed circuit board 10 is used as the common ground for the radio circuit block 21 and digital circuit block 22. The printed circuit board used herein includes a first layer 11 as a signaling layer, a second layer 12 as the ground layer, a third layer 13 as a power source layer, and a fourth layer 14 as another signaling layer, which are disposed in this order as viewed from the first layer 11 mounting thereon components. It is to be noted that only some of circuit patterns 24 are depicted on the third and fourth layers for simplification purpose (FIG. 21C). Generally, the spaces between these layers are filled with a dielectric material (not shown) such as a glass epoxy material.
In the wireless portable device as described above, if there are a plurality of transmission channels provided in the same circuit board, and the distance between adjacent transmission channels is short, interaction therebetween may be generated to cause an electromagnetic coupling. An example of the methods which can solve such a problem is described in Patent Publication JP-A-58-092101. In this method, metallized through-halls are provided between the transmission lines, which should be isolated from one another, for connecting together the upper ground conductor and the lower ground conductor, whereby electric isolation is attempted between adjacent transmission lines.
In Patent Publication JP-A-10-75108, a dielectric waveguide-tube transmission line is described which is formed by the area encircled by two arrayed via-hall groups which electrically connect together the conductor layers. This technique features a subordinary conductor layer formed parallel to the conductor layers and electrically connected to the via-holes. It is attempted to improve the transmission characteristic therein by introducing such a subordinary conductor layer. Similar conductor via-halls are used in Patent Publication JP-A-9-46008. This publication describes the technique wherein the length of a stub located on the periphery of the ground pattern is made to be less than ¼ of the wavelength of the high-frequency signal transmitted through the signal transmission line. The term “stub” as used herein means an edge that is not electrically connected to the ground pattern directly. By adopting such a configuration, a high-frequency-wave wiring board can be obtained having a reduced influence on the high-frequency signals transmitted through the signal lines.
It is to be noted that a structure such as shown in FIGS. 22A and 22B may be used for restricting the current flowing on the cable. FIG. 22A is a perspective view, and FIG. 22B is a sectional view taken at the central plane of the cable. In this configuration, one of the ends of a metallic hollow cylinder 41 is short-circuited by a metallic plate 42, and the metallic hollow cylinder 41 is disposed to cover the cable 43. The length Lc of the hollow cylinder is set at ¼ of the wavelength of the current transferred through the tube. In this example depicted, the short-circuiting plate is connected to the right end to form an electrically short-circuited plane, whereas the left end constitutes an open plane. In general, the position apart from the short-circuited plane by ¼ of the wavelength is an open plane (open end), wherein the input impedance Zin at the position of the open plane as viewed from the A-side in the drawing assumes a higher value. Accordingly, the current I flowing from the A-side toward the B-side is suppressed by the effect of the higher impedance of the open plane of the structure depicted in these figures, if it is provided therebetween.
In general, the frequencies of the signals to be handled are different between the radio circuit block and the digital circuit block. For example, the radio circuit block handles transmitted/received signals having frequencies around 1 GHz (may be around 2 GHz instead, depending on the device). On the other hand, the digital circuit block handles a clock signal having a fundamental wave of around 10 GHz, which generates higher-harmonic frequencies equal to the integral multiples of the frequency of the fundamental wave. Thus, the ground layer common to both the circuit blocks receives thereon a mixture of the transmitted/received signals of around 1 GHz (or around 2 GHz) generated from the radio circuit block and the fundamental-wave and the higher-harmonic-wave signals generated from the digital circuit block. As a result, there is a tendency that the radio circuit block and the digital circuit block are susceptible to the influence by the electromagnetic coupling due to the signals of each other.
For example, it is probable that the higher-harmonic-wave current generated in the digital circuit block and transferred through the ground layer enters a device, such as the IC, in the radio circuit block. On the other hand, it is also probable that the high-frequency current (radio-frequency-wave current) around 1 GHz generated in the radio circuit block enters the digital circuit block. In the wireless portable device having reduced size and thickness, it is general that the radio circuit blocks and the digital circuit blocks are mixed on a single circuit board in a closed relationship therebetween. Thus, the electromagnetic coupling generated between the radio circuit block and the digital circuit block tends to become more critical. It has been desired to effectively suppress the electromagnetic coupling between the radio circuit block and the digital circuit block for assuring a reliable quality in such a wireless portable device. In the above conventional techniques, there is no teaching to effectively solve the problem while noting the electromagnetic coupling of the signals having different frequencies between the radio circuit block and the digital circuit block.